This invention relates to an improved method of concentrating by multiple effect evaporation solutions containing mixtures of salts and particularly this invention relates to concentrating by multiple effect evaporation solutions containing a first and a second salt where the solubility of the first salt increases more greatly with increasing temperatures than the solubility of the second salt. Even more particularly, this invention relates to an improved method whereby the solution concentrated contains principally potassium chloride and sodium chloride with minor amounts of other salts such as magnesium chloride, calcium sulfate, calcium carbonate, magnesium sulfate, and calcium chloride.
Salts having a solubility that increase with increasing temperatures within a given temperature range, hereinafter called first salts, and salts having a solubility that remains relatively unchanged or decreases with increasing temperatures within the same temperature range, hereinafter called second salts, are frequently found as mixtures in naturally occurring ores. In recovering such salts, the ore is dissolved in a suitable aqueous solvent forming a solution from which the salts can be easily separated. Admixtures of these salts in solution can also arise as a result of commercial chemical production, e.g., electrolysis of electrolytes. Conventionally, salts are separated from the solution by concentrating the solution by evaporation to produce a solution in which the first and second salt are concentrated to their "invariant composition". By "invariant composition" is meant a composition in which a solution at a given temperature is saturated with respect to two or more salts. This solution is forwarded to a recovery zone where the first salt is recovered, e.g., by cooling the solution so that the first salt will crystallize out of solution and precipitate. The temperature range at which the water removal step and cooling step take place is a range in which the first salt and second salt maintain their solubility characteristics. If the second salt is initially in high enough concentration, it will be precipitated and can be recovered during the initial evaporation step. Otherwise, subsequent evaporation after first salt is recovered can yield production of the second salt. Thus, alternate evaporation and cooling can substantially deplete the solution of the first and second salt.
Potassium chloride (first salt) and sodium chloride (second salt) are recovered commercially from naturally occurring ores comprising principally potassium chloride and sodium chloride and minor amounts of magnesium chloride, calcium chloride, magnesium sulfate, calcium sulfate, and calcium carbonate, i.e., less than 2 percent of other salts and impurities. In this commercially practiced process, water is removed from the solution by evaporation until the solution approaches or reaches its invariant composition. Large amounts of sodium chloride and some salt impurities are precipitated and sodium chloride removed during the evaporation step. The solution is then purged of impurities precipitated during evaporation and cooled to crystallize potassium chloride while other salts and impurities remain in solution.
The invariant composition of potassium chloride-sodium chloride solutions is affected by other salts in the solution. For example, solutions of many naturally occurring potassium chloride-sodium chloride containing ores also contain chloride, carbonates, and sulfates of anions other than sodium and potassium as hereinbefore mentioned. The presence of some of these other salts will lower the salt concentration of the invariant composition from the concentration found for a mixture of only sodium chloride and potassium chloride. For example, the presence of a few parts magnesium chloride per hundred parts water will lower the invariant composition by a few parts each of sodium chloride and potassium chloride per hundred parts of water.
Evaporation of potassium chloride-sodium chloride solutions are carried out with great efficiency by utilizing evaporators in a multiple effect manner to achieve high product recovery and great steam economy. These multiple effect evaporators operate at relatively higher temperatures in the direction of the flow of the solution. That is, mother liquor effluent overflow from cooler evaporator effects is forwarded to hotter evaporator effects. To obtain a satisfactory working temperature difference between the first evaporator effect and the last evaporator effect, the first evaporator effect is operated under super atmospheric pressure and the last evaporator effect is operated under vacuum. As the solution passes through each evaporator effect, water is removed in the form of vapor and the solution becomes concentrated with respect to the salts; thus, sodium chloride will begin to precipitate (because of its solubility characteristic) and settle to the bottom of the evaporator effect where it is recovered, e.g., through an elutriation leg in communication with the bottom of each evaporator. Sodium chloride will precipitate until the solution reaches its invariant composition for the temperature at which each evaporator effect is operated. Impurities having second salt solubility characteristics such as calcium sulfate, calcium carbonate, and magnesium sulfate may be precipitated as well during the process but are fluidized to avoid their settling.
These evaporator effects are commonly heated by steam in a direction opposite to the direction of the flow of the solution to be concentrated. The first evaporator effect is heated by introducing steam from an external source, such as a boiler, and the second evaporator effect is heated with the vapors from the first evaporator effect and so on, progressively to the last evaporator effect.
Mother liquor effluent overflow from the first evaporator effect is transferred to a potassium chloride recovery zone. This mother liquor effluent overflow is usually slightly below the invariant composition, that is, less than 100 percent saturated with respect to potassium chloride (above 85 and up to about 98 percent saturated with respect to potassium chloride) so that potassium chloride is not crystallized out and lost before reaching the potassium chloride recovery zone. The recovery zone can be a series of crystallizers in which the mother liquor effluent overflow from the first evaporator effect is cooled to precipitate potassium chloride. Due to the hereinbefore described solubility characteristic of sodium chloride (a second salt), sodium chloride will not precipitate thereby allowing potassium chloride to precipitate esentially pure.
Before being introduced to the first salt recovery zone, the first evaporator effect mother liquor effluent overflow is commonly forwarded to a solids settling zone or thicknener. In the solids settling zone, fine particles comprising mostly precipitated fluidized salt impurities, as hereinbefore described, are allowed to settle. Typically, the settling zone is operated at atmospheric conditions and must be maintained under quiescent conditions in order for the settling to take place. Once the finely divided particles have settled out of the mother liquor, the mother liquor can be forwarded to the potassium chloride recovery zone. It is therefore imperative that the first evaporator effect mother liquor overflow leaving the first evaporator effect at super atmospheric pressure and above its atmospheric boiling point temperature does not experience flashing in the settling zone. Flashing will result in agitation of the mother liquor in the settling zone, thereby making it difficult for the settling of the fine particles to occur. Also, flashing cools the mother liquor causing potassium chloride to be precipitated and consequently lost with the settling solids which are purged from the process.
The mother liquor effluent can be flashed into a special flash evaporator from which the vapor is recycled to the multiple effect evaporation zone and the equilibrium liquid forwarded to the settling zone under quiescent conditions. This method is undesirable because fine particles of potassium chloride crystals are precipitated in the flash evaporator along with sodium chloride thus requiring recycle of the solids back to preceding evaporators for the potassium chloride to be redissolved. Moreover, this method is undesirable because the energy level or temperature of the flashed vapor is often not sufficiently high to be used above the third evaporator effect thereby losing one effect of evaporator economy based on a thermal energy balance.